In case any issue with the plugin or messenger – please check these basic troubleshooting things.
Whitelisted Domains
Whitelisted the domain name at Facebook page settings ( please check again, If you give access, remove access to third-party tools, they may clear the whitelisted domain names list )
Facebook page is Publicly Visible
Make sure that Facebook page is publicly visible and not resisted by age, country. and make sure other users can send message.
If age, country restriction are added to the Facebook page, messenger may visible only to the users who logged in to Facebook in that browser.
( check that non admin user can send message to your Facebook page m.me/your_page_id or try to send message to others, from your Facebook page messenger )
HTTPS
Make sure Website is Served over HTTPS.
It may work even if the website is not loading from https, but at whitelisted domains section add website with http also.
Mulitple Facebook SDK are added
If multiple Facebook SDK are added, it may not work as expected. Especially When both sdk are in different languages or in different version. In this case – try uninstall the other source that add’s Facebook sdk and test ).
Cache
Sometimes cache plugins installed on website may not updated according to the settings. clear the cache created by cache plugins.
If mobile alone its not working, then it might be browser cache, clear the cache from the browser and test.
Add correct page id in Plugin Settings page
( Find Facebook Page ID )
Referrer-Policy
make sure Referrer-Policy header is not set to no-referrer.
WP-Chabot plugin pass the values based on your plugin settings.
Most of the things will be handle by Facebook SDK ( loading Messenger, Styles … ).
In case any issue with the plugin, please create a new topic in plugin support forum.
In case any issue with the Facebook related things – please report it to Facebook team
https://developers.facebook.com/bugs/
https://developers.facebook.com/bugs/create/